This invention was developed by BaySaver Technologies Inc. (BaySaver) a manufacturer of stormwater pollution controls based in Maryland. BaySaver is licensed under U.S. patents on several different physical separators, including patents U.S. Pat. Nos. 5,746,911 and 6,264,835, both titled “Apparatus for separating a heavy fluid from a light fluid.” While physical separators accomplish a great deal and are an important factor in environmental protection, they do not remove dissolved contaminants, nor do they remove sediment particles in the fine silt and clay size range.
Filtration is a proven method of removing dissolved and very fine impurities from a fluid. The most commonly used filter in the stormwater industry is a depth filter. A depth filter uses a media (sand, for example) that the fluid must pass through. The removal is achieved by a combination of two mechanisms: transport and attachment. In a sand filter, the individual grains obstruct the flow of water, forcing the fluid to take a more tortuous path through the filter. When this happens, the fluid comes into contact with far more of the media than it would if it were to flow straight through the filter. The fluid, along with the impurities to be removed, is in contact with the individual grains of sand or other filter media. Bringing the contaminants into contact with the filter media is the first stage—the contaminants are transported into contact with the filter media.
In order to remove the pollutants from the fluid, the media must have some way of capturing and retaining the contaminant. This can be accomplished in a number of ways, from chemical bonding on reactive media to simple sedimentation in the interstices between the media components. When designed carefully, media filtration is capable of removing large pollutant loads from influent fluid streams. The present invention can be used to remove fine sediments, and other entrained pollutants such as oils and grease, and dissolved contaminants from runoff streams. It can also be used to reduce turbidity in the same applications.
The selection of filter media can depend on many factors, including the type of contaminants targeted for removal, the desired flow rates through the filter, the cost and weight of the media, and other factors. While filtration is a proven technology in fields like wastewater treatment, drinking water treatment and industrial processes, it is relatively new in the field of stormwater treatment.
One of the challenges of stormwater management is dealing with the highly variable runoff flow rates, that result from storms. In stormwater applications, treatment devices are subjected to widely varying flow rates, from very slow trickles to the runoff resulting from torrential downpours. To accommodate these variations, many stormwater treatment devices are designed with an internal or external bypass. The bypass allows flows in excess of the intended treatment capacity to pass through the unit untreated, while continuing to treat flows within the intended range. This is a viable technology because of the “first flush” effect in stormwater runoff. During the beginning of a storm event, a “flush” of contaminants is carried off of a site with the first bit of runoff. This flush includes the pollutants that have collected on that site since the last storm, and comprises the majority of the pollution load from each storm event. If the first flush is treated adequately, a majority of the potential contaminants will be removed during that treatment and the subsequent bypass flows during extreme storms will not contain the same heavy pollutant loads.